The plant - which began operations in 1987 - is located in New Hill, about 20 miles southwest of Raleigh.
Officials said the quarter inch crack was not all the way through the reactor vessel wall and there is no indication any radioactive material escaped. They say the decision to shut down the reactor and repair was a conservative one.
However, Duke admits the information they used to see the crack was collected during an inspection a year ago.
The Nuclear Regulatory Commission told ABC11 that Duke Energy was preparing for an upcoming maintenance outage and re-reviewed images taken last year when they noticed the crack in a weld.
"We're going to evaluate the data again, but we do multiple evaluations of the data and made this conservative decision to the take the unit off line to make the repair," said Duke Progress Energy's Kim Crawford.
A report on the NRC's website said there is no impact to the health and safety of employees or the public.
Meanwhile, Jim Warren with NC WARN said taking the plant off-line unexpectedly will cost $1 million a day. It's money he believes Duke Energy will try to make up for with a rate increase.